1. Technical Field
The present invention relates to a fiber-optic gyroscope and a unit thereof, and more particularly to a compact three-axis fiber-optic gyroscope and a unit thereof.
2. Background Art
A sensor which is simply mounted on a moving body and is capable of detecting a rotational angular velocity or angle without any external information, to enable control of the attitude or the position of the moving body is generally called "gyroscope" or "gyro". Fiber-optic gyroscopes are gyroscopes used to detect rotational velocities or angles by use of the Sagnac effect. In recent years, single-axis fiber-optic gyroscopes have come to be used in the industrial field because of their following characteristics:
(1) No movable parts and simple construction; PA1 (2) Short starting time; PA1 (3) Can be constructed in a smaller size with lighter weight; PA1 (4) Small power consumption; PA1 (5) Wide dynamic ranges; and PA1 (6) Suitable for mass production and cost reduction.
A block diagram of a typical fiber-optic gyroscope is shown in FIG. 4 of the accompanying drawings. The illustrated fiber-optic gyroscope includes a light source 11, a light receiver (detector) 12, an optical fiber coil 18, a phase modulator 14, directional couplers 15a and 15b, a polarizer 17 and a circuit board 16 with a signal processing circuit mounted thereon. A light beam emitted from the light source 11 passes through the directional coupler 15a. After polarization noises are removed from the light beam by the polarizer 17, the light beam is split by the directional coupler 15b into two light beams, which is conducted along two optical paths, and these two light beams propagate through the optical fiber coil 18 as a clockwise beam and a counterclockwise beam, respectively. The optical fiber coil 18 is a coil of optical fiber which functions as an angular velocity sensor of the fiber-optic gyroscope. The rotation of the fiber-optic gyroscope 18 causes a phase difference between the clockwise beam and the counterclockwise beam, and the amount of rotation can be known by detecting the phase difference. Therefore, there is obtained only the amount of rotation about the center axis of the fiber-optic gyroscope 18, namely, only the amount of rotation around one axis can be known. The phase modulator 14 operates to give a phase bias of .pi./2 between the clockwise and counterclockwise beams, to realize a desired sensitivity. The clockwise and counterclockwise beams propagating through the optical fiber coil 13 join at the directional coupler 15b and become an interference beam due to the phase difference therebetween. Then, the interference beam is so directed as to pass through the polarizer 17 and the directional coupler 15a, in a direction reverse to the passage of the above emitted light beam, and introduced into the light receiver (detector) 12. In the light receiver 12, the intensity of the interference beam is measured and the intensity is converted into an amount of rotation by the signal processing circuit mounted on the circuit board 16. Generally, circuits for driving the light source 11 and the phase modulator 14 are also mounted on the circuit board 16.
However, the single optical fiber coil can only detect the amount of rotation about a single axis, as described above. Therefore, three optical fiber coils are required to detect the attitude of an object which moves in three dimensions.
In addition, these optical fiber coils should have their center axes arranged orthogonal to each other. When three optical fiber coils for the three axes respectively are separately installed, a large installation space is necessary. In order to reduce the space, the conventional optical fiber coils of the fiber-optic gyroscope have been formed in different sizes as shown in FIG. 5. Specifically, the smallest coil 23 is installed in the medium coil 22 and the medium coil 22 is installed in the largest coil 21. In this construction, however, the assembling is not easy and mass production becomes difficult.
On the other hand, when the three single-axis fiber-optic gyroscopes (generally each gyroscope has a rectangular parallelopiped outer shape and includes the above mentioned type of optical components and signal processing circuitry) simply connected together, the whole system becomes very large.
Japanese Patent Application Publication, Publication No. 61-266911, titled "Surveying Instrument Using Optical Fiber Gyro", published Nov. 26, 1986, owned by Nippon Kogaku Kabushiki Kaisha, discloses a gyroscope similar to one illustrated in FIG. 5 and a U.S. Pat. No. 4,893,930 issued to Garret et al, Jan. 16, 1990 discloses a "Multiple Axis, Fiber Optic Interferometric Seismic Sensor".